Various processes involving Injection molding have been disclosed in the prior art for the fabrication of plastic lenses. In a typical process for the injection-molding of plastic lenses, plastic resins, such as polycarbonate (PC) or polymethylmethacrylate (PMMA), are initially heated to a molten state. Then, the molten plastic is injected into a mold cavity so that it assumes the compensated form of the mold cavity when it cools. Empirically, good lens quality is achievable if the injection molding process is used in the production of relatively thick lenses (including both the concave lenses and convex lenses). However, it has not been successful to use the injection molding process to fabricate high-quality relatively thin lenses, particularly, thin concave lenses.
A mold cavity for the fabrication of concave lenses has a relatively thinner central portion and a relatively thicker annular portion (i.e., the annular portion is thicker than the central portion). During the conventional injection-molding process, the molten plastic flows through the annular portion of the molding cavity at a greater linear speed than the speed by which it flows through the central portion of the same. As a consequence, the molten plastic diverges into two melt fronts (leading tips of the molten plastic) along the annular portion of the mold cavity. When the two melt fronts meet at a seam, a weld (knit) line is formed on the surface of the lens being formed. For plastic concave lenses having the same diameters, the higher the degree of the concave lens, the longer the weld line of the concave lens would become. The weld line is formed because the melt fronts of the molten plastic cool and thus at least partially solidify when they flow towards each other. The solidification prevents the two melt-fronts to satisfactorily merge with each other. One way to solve this problem is to externally apply heat to the mold to maintain the mold at a temperature such that the viscosity of the plastic is retained at a minimum. However, this introduces another problem in that this process would require a relatively longer period of time for the molten plastic to cool and solidify. As a consequence, the plastic lenses cannot be produced at high rates using the conventional injection-molding process.
Another way to alleviate the above-mentioned problem is to compression-mold blanks of lenses (i.e., precursory lenses) into finished lenses. A typical process for compression-molding of plastic lenses employs a mold having a sleeve and two inserts (dies) slidably mounted in the sleeve. The confronting tips of the inserts define the compensated form of the surfaces of the lens to be manufactured. The sleeve has a tubular inner wall. A mold cavity is defined by the confronting tips of the inserts and the inner wall of the sleeve. The blanks are preheated to their melting temperature and placed inside the mold cavity. The inserts are moved towards each other so as to compress the blanks. The blanks, after compression, assume the compensated form of the mold cavity. However, since it is difficult to prepare blanks having exactly the same amount of plastic material required for making the desired lens, it is difficult to have finished lenses having exactly the desired dimensions. The conventional process requires that the mold to be maintained at or near the glass transition temperature of the plastic being molded so that the molten plastic is maintained in a viscous form. Because of the accompanying low heat transfer coefficients associated with the highly viscous material, it takes a long period of time for the plastic to cool and solidify. Consequently, with the conventional compression-molding process, the lenses also cannot be produced at satisfactorily high rates.
There have been several attempts in the industry to combine injection molding and compression molding for the production of plastic lenses. U.S. Pat. No. 4,091,057 issued to Hermann P. Weber on 23 May 1978 teaches a method for the injection molding of plastic lenses. This method employs a mold having at least an upper inner block defining a hole, a lower inner block defining another hole, an upper insert slidably mounted in the upper inner block, a lower insert slidably mounted in the lower block, several heating rods mounted about the upper and lower inner blocks, and a heating rod axially mounted in the lower insert. The confronting tips of the inserts define the compensated form of the surfaces of the lens to be produced. Each of the upper and lower inner blocks defines a tubular inner wall. The confronting tips of the upper and lower inserts and the inner walls of the upper and lower inner blocks define a mold cavity having a central portion and an annular portion having a thickness greater than the central portion. The lower inner block also defines an inlet in communicating with the hole formed in the same. The lower inner block also defines an overflow passageway, by means of which the hole formed in the lower inner block can be communicated to an overflow pocket formed in the lower inner block. A piston is slidably received in the overflow pocket and biased by a spring attached to the mold so that the volume of the overflow pocket is adjustable. A thermoplastic material is heated to a temperature of 520.degree.-560.degree. F. so that it is molten in order to be injected into the mold cavity, which is heated to a temperature of 260.degree.-275.degree. F. by means of the heating rods. The force of the molten plastic against the upper insert causes the upper insert to rise, thereby increasing the thickness of the central portion of the mold cavity. Thus, a weld line will not occur. Then, the upper insert is moved towards the lower insert in order to compress the molten plastic which is urged into the overflow pocket through the overflow passageway. In this manner finished lenses with predetermined dimensions are obtained. In the process disclosed in the '057 patent, the injection of the molten plastic material into the mold cavity is stopped before the compression of the molten plastic in the mold cavity. That is, the melt front pauses before it is urged into the overflow pocket. Birefringence thus occurs at the point wherein the melt front pauses.
U.S. Pat. No. 4,364,878 issued to Albert J. Laliberte on 21 Dec. 1982 teaches a method for molding ophthalmic lenses. Such a method employs a mold having two sleeves and two mold inserts slidably mounted in the sleeves. The sleeves each define a tubular inner wall. The confronting tips of the mold inserts define the compensated form of the surfaces of the lenses to be manufactured. The confronting tips of the mold inserts and the inner walls of the sleeves define a compressible mold cavity. Initially, the mold inserts are positioned at an extended distance from each other so that the mold cavity is greater than in volume than the desired lenses. Then, molten plastic is injected into the mold cavity. Thereafter, the mold inserts are moved to an appropriate distance from each other so that the mold has exactly the compensated form of the desired lenses. Finally, the lenses are cooled and allowed to solidify and assume the shape which is the compensated form of the mold cavity. With the method disclosed in the '878 patent, the amount of plastic injected into the mold cavity must be exactly the same as the amount of plastic required for the desired lenses, as the mold does not define any overflow cavity. This requires a measuring device for ensuring that exactly the amount of molten plastic is injected into the mold cavity. To satisfy this requirement, molten plastic is injected into the mold cavity from a reservoir through the measuring device. The measuring device must be heated so as to maintain the molten plastic at a certain temperature so that the molten plastic is retained at a certain viscosity in the measuring device in order for the measuring device to properly function. Therefore, this process requires more energy. Furthermore, it takes a longer period of time as it involves the extra steps of measuring the quantity of the molten plastic. It takes an even longer period of time as it involves a long travel of the molten plastic and birefringence also occurs as a consequence.
U.S. Pat. No. 4,836,960 issued to David P. Spector et al. on 6 Jun. 1989 teaches a fabrication of plastic optical components by injection/compression molding. The fabrication process disclosed in the '960 patent employs a mold having a sleeve and two mold dies slidably mounted in the sleeve. The sleeve defines a tubular wall. The confronting tips of the mold dies define the compensated form of the surfaces of the desired lenses. The inner wall of the sleeve and the confronting tips of the mold dies define a compressible mold cavity. Initially, the assembly is heated to a temperature above the glass transition temperature of the thermoplastic material to be molded. An injection port extends through the sleeve to the bore, and is positioned to inject the thermoplastic material that has been heated to a liquid state into the cavity. After injection of the thermoplastic material, the mold inserts are moved toward each other to compress the thermoplastic material, while excess thermoplastic material is forced out of the mold cavity. The molds are then turned in a reverse direction to uncouple the injection port from the cavity. The assembly is then cooled to a temperature below the glass transition temperature of the thermoplastic material. Finally, the lens is removed from the mold. The method disclosed in the '960 patent has the disadvantage that, because it requires both the heating and cooling steps, it takes a long period of time to complete the process. Furthermore, expensive injection molding apparatus is required to practice the process disclosed therein, particularly to accommodate the heating and cooling requirements.